This invention relates to the production of patterned abrasive surfaces on substrates in a form useful for fine finishing of substrates such as metals, wood, plastics and glass.
The proposal to deposit isolated structures such as islands of a mixture of a binder and abrasive material on a backing material has been known for many years. If the islands have very similar heights above the backing and are adequately separated then, (perhaps after a minor dressing operation), use of the product will result in reduced surface scratching and improved surface smoothness. In addition the spaces between the islands provide a route by which swarf generated by the abrasion can be dispersed from the work area.
In a conventional coated abrasive, investigation of the grinding surface reveals that a comparatively small number of the surface abrasive grits in an active abrading zone are in contact with the workpiece at the same time. As the surface wears, this number increases but equally the utility of some of those abrasive grits may be reduced by dulling. The use of abrasive surfaces comprising a uniform array of isolated islands has the advantage that the uniform islands wear at essentially the same rate such that a uniform rate of abrasion can be maintained for longer periods. In a sense the abrading work is more evenly shared among a larger number of grinding points. Moreover since the islands comprise many smaller particles of abrasive, erosion of an island uncovers new, unused abrasive particles which are as yet undulled.
One technique for forming such an array of isolated islands or dots that has been described is that of the rotogravure printing. The technique of rotogravure printing employs a roll into the surface of which a pattern of cells has been engraved. The cells are filled with the formulation and the roll is pressed against a surface and the formulation in the cells is transferred to the surface. Normally the formulation would then flow until there was no separation between the formulations deposited from any individual cell. Ultimately a layer of essentially uniform thickness would be obtained. By way of illustration, comparative Examples C and D of U.S. Pat. No. 5,152,917 describe a process in which the pattern obtained by a rotogravure process quickly lost all separation of the individual amounts deposited from the cells.
In U.S. Pat. No. 5,014,468 a binder/abrasive formulation was deposited from rotogravure cells on a roller in such a way that the formulation was laid down in a series of structures surrounding an area devoid of abrasive. This is believed to be the result of depositing less than the full volume of the cell and only from the perimeter of each cell, which would leave the ring formations described.
The problem with the rotogravure approach has therefore always been the retention of a useful shape to the island. To formulate an abrasive/binder mixture that is sufficiently flowable to be deposited and yet sufficiently non-flowable such that it does not slump to an essentially uniform layer coating when deposited on a substrate has proved very difficult.
Chasman et al., in U.S. Pat. No. 4,773,920 disclosed that using a rotogravure coater, it is possible to apply a uniform pattern of ridges and valleys to the binder composition which, when cured, can serve as channels for the removal of lubricant and swarf. However beyond the bare statement of possibility, no details are given that might teach how this might be carried out.
In U.S. Pat. No. 4,644,703 Kaczmarek et al. used a rotogravure roll in a more conventional fashion to deposit an abrasive/binder formulation to deposit a layer that is then smoothed out before a second layer is deposited by a rotogravure process on top of the smoothed-out first layer. There is no teaching of the nature of the final cured surface.
In U.S. Pat. No. 5,014,468 (Ravipati et al.) it was proposed to use an abrasive/binder mixture having non-Newtonian flow properties and to deposit this mixture by a rotogravure technique on to a film. In this process the mixture was deposited from the edges of the rotogravure cells to produce a unique structures with deposits of reducing thickness with distance away from the surface surrounding areas devoid of the mixture. If the cells are sufficiently close together, the surface structures can appear interlinked. This product has proved very useful, particularly in ophthalmic fining operations. The process is very useful but it has a potential problem with increasing build-up of material in the cells of the rotogravure roll such that the deposition pattern can change slightly during a protracted production run. In addition the nature of the process is such that it is limited to formulations containing relatively fine abrasive grits, (usually less than 20 microns).
Another approach has been to deposit the abrasive/binder mixture on a substrate surface and then impose a pattern comprising an array of isolated islands on the mixture by curing the binder while in contact with a mold having the inverse of the desired patterned surface. This approach is described in U.S. Pat. Nos. 5,437,754; 5,378,251; 5,304,223 and 5,152,917. There are several variations on this theme but all have the common feature that each island in the pattern is set by curing the binder in contact with a molding surface. This approach too is not without its problems in that incomplete pull-out from the mold often occurs such that, instead of producing, for example pyramids, volcano shapes complete with crater, frequently result.
The present invention presents a technique for producing uniformly patterned shapes of an abrasive/binder combination that does not require a cure-in-mold operation or the selection of a binder/abrasive combination with specific non-Newtonian flow characteristics.
The present invention therefore provides a flexible and effective route for the commercial scale production of coated abrasives with a uniform array of isolated abrasive composite shapes. Such coated abrasives are well adapted to the treatment of a wide range of substrates to yield fine finishes for protracted periods of operation at a substantially uniform cut rate.